AIMS: Diabetic patients without clinical evidence of cardiovascular disease may develop left ventricular (LV) dysfunction. The present study was designed to test the hypothesis that coronary microvascular dysfunction affects LV function in type 2 diabetic patients. METHODS: The study subjects were 20 type 2 diabetic patients and 15 controls, who had been angiographically determined to have normal coronary arteries. LV ejection fraction (LVEF) and the percentage change in LVEF during dobutamine infusion (DeltaLVEF) were measured as an index of LV function. In order to evaluate coronary flow reserve, coronary flow velocity was recorded using a Doppler guide wire. RESULTS: There were no significant differences in LVEF. DeltaLVEF was significantly lower in the diabetic patients than in the control subjects (p<0.01). Although there was no significant difference in the baseline coronary volumetric flow between the two groups, values during maximal hyperemia were significantly less in the diabetic patients than in the controls (p<0.05). Consequently, coronary flow reserve was significantly lower (p=0.0001). A significant positive correlation between coronary flow reserve and DeltaLVEF was exhibited (r=0.68, p=0.0009). CONCLUSIONS: Coronary flow reserve, an indicator of coronary microvascular function, is significantly reduced in type 2 diabetes, and this is reflected in impairment of myocardial contractile reserve.